Leaking underground storage tanks can be dangerous and costly. Where the tanks store fuel, leaks can mean loss of such valuable fuel and it can also mean contamination of surrounding soil and community drinking water supplies. At the present time, nominal leakage and product loss from fuel storage is usually acceptable from an operational standpoint, but is not acceptable for potential long-term environmental damage for the reasons stated above. Due to metering accuracy limitations, and the volumetric changes of petroleum products with temperature, inventory control data is usually inadequate for determining the existence of leaks. Therefore, verification of tank integrity requires the use of more precise measurement methods or monitoring procedures.
Several conditions can occur which require tests to determine the tightness of underground storage tanks and piping. Among the situations where such tests should be conducted are: in the course of a tank upgrading program; where there is a suspicion of a leak because of stock inventory loss; when leak monitoring indicates ground contamination; when there is an accumulation of water in the tank; and upon completion of construction.
Although many factors may cause an underground tank to leak, corrosion of the tank and piping have been determined to be the principle causes.
There are several typical leak locations in underground storage tanks, and several factors contribute to leaks occurring from these storage tanks. External tank corrosion is the greatest single factor leading to leaks. One of the major causes of tank corrosion is the corrosive potential of the soil caused by long-term location of unprotected underground steel takks in areas with high soil moisture content. Electrical conductivity of the soil appears to be one of the primary factors involved. Soil moisture content is an important condition which affects soil electrical conductivity and should be given special consideration in areas subject to high water tables, tidal fluctuations and high rainfall which may create high corrosion potential. Soil with a higher content of dissolved solids and salinity will also result in higher conductivity and therefore greater corrosion potential. In addition to soil moisture content, other site-specific factors such as buried metal objects and natural variations in soil characteristics, such as pH, can also contribute to the electrical potential at a given site. An additional factor is that lack of oxygen in the soil adjacent to the tank can enhance the growth of anaerobic bacteria which accelerates the corrosion process.
Improper installation practices can also lead to underground storage tank leakage and this would include improper preparation of the excavation pit which could result in tank rupture due to settling or uneven loads. Additionally, abrasion of the tank surface during installation can greatly increase the rate of localized corrosion.
Physical damage of the inside of the tank as a consequence of such factors as faulty management operations and inventory control operations can also contribute to leakage from underground storage tanks. An example of this is repeated contact at the same point in a tank by a dipstick while taking product level measurements, which can result in a weak spot in the tank.
Another contributing factor to possible leakage is age of the tank.
While fiberglass tanks are sometimes used, the majority of tanks are constructed of steel. This is because steel offers substantially lower installation costs. However, a substantial number of fiberglass tanks are in use and have significant potential for developing leaks. Installation stresses and settling are often the cause of leaks developing in these types of tanks.
It is important to realize that the piping associated with a tank, normally located at a level above the top of the tank, is often the location of storage tank leaks.
The current industry standard for threshold detection of leakage has been established by the National Fire Protection Association (NFPA) as 0.05 gallons per hour, regardless of tank size.
In detecting leakage, the impact of geometry is significant for large tanks. A small diameter but very tall tank is relatively easy to monitor since a small volume change will produce a relatively large liquid level change. However, most large storage tanks are constructed with large horizontal dimensions and limited height for a number of practical reasons. This means that large volume change in such a tank will produce a small change in liquid level, tending to reduce the accuracy of any computations based on level measurements.
Typical tank leakage monitoring systems currently available have a practical tank size limitation of approximately 20,000 gallons for the NFPA standard of detection. Because of the problems associated with geometry and total volume for larger tanks, a substantially higher threshold detection level for these tanks can still provide valuable information and also might be all that one could determine from present systems.
One of the major problems encountered in testing underground storage tanks is operator error. Several studies have identified and confirmed this error source. For example, with most previously available leak testing systems, the operator can incorrectly set up the equipment, misread data or can make leak rate calculation errors. The most likely but worst case result is a reported leak rate having errors that are undetectable.
Many of the systems presently available are relatively complex and require trained people who are very precise in all phases of the tank monitoring procedure. Also, many different types of devices are available, most of which measure absolute temperature and level changes in some way. One system works on the principle of buoyancy where the vertical position of a floating container is used in calculating volumetric changes. Others use sonic techniques, bubbling techniques, and still others use helium as a trace gas to detect leaks.
Most of the presently available leak monitoring systems for large storage tanks have one or more significant drawbacks. Many prior art systems require absolute temperature measurement, which is subject to substantially greater error than would result from measurement only of change in temperature, which is all that is really of interest. Additionally, large, cumbersome and relatively complex materials and equipment are necessary for many of the present systems. Further, skilled operators are frequently necessary and, as stated above, operator interpretation can lead to significant costs and errors.